A drilling machine having an electric drill supported by a frame which includes an electromagnet(s) in the bottom is known. In drilling operation, the frame is securely anchored to a workpiece by the action of the electromagnet. A common procedure of the drilling operation of the drilling machine will be described. The procedure starts with positioning the frame with the electromagnet on the workpiece and energizing the electromagnet to secure the drilling machine to the workpiece by its electromagnetic force. Then, the motor of electric drill is energized to drill a hole in the workpiece. The electric drill may be moved downward or forwarded by either manual action with a handle or automatic feeding with a motor.
A conventional controlling apparatus for energizing the electromagnet and the drill motor is shown in FIG. 4, in which an AC power source 3 is connected between two terminals 1 and 2. The AC power source 3 is connected via a main switch 4 to a rectifier 6 of which output is connected to a coil 7a of the electromagnet. Also, the AC power source 3 is connected 8 to a motor 9 of the electric drill via the main switch 4 and a normally open contact 8a of a relay. It is further connected by a drill start switch 5 to the relay 8 and a stop switch 11. The drill start switch 5 is connected in parallel with another normally open contact 8b of the relay 8.
In action, when the main switch 4 is closed, a voltage from the AC source 3 is rectified by the rectifier 6 to be applied to the coil 7a of the electromagnet for magnetization. After confirming that the drilling machine is securely anchored to a workpiece by the action of the electromagnet, the operator presses down the drill start switch 5 to actuate the relay 8. When the two contacts 8a and 8b of the relay 8 are closed, the drill motor 9 starts rotating for carrying out the drilling action. Once the drill start switch 5 is pressed, the contact 8b remains closed to maintain the state of the relay 8. To stop the electric drill, the stop switch 11 is pressed to disable the relay 8. For removing the drilling machine from the workpiece after completion of the drilling operation, the main switch 4 is turned off to deenergize the coil 7a.
The conventional drilling machine allows the motor 9 to stop immediately when the energization is interrupted by an abrupt fault such as power failure or accidental unplugging during the drilling operation. Since the power interruption causes the relay 8 to open its contacts, the electric drill is prevented from unexpectedly starting again after the fault is eliminated and the source power is recovered. More specifically, when the connection to the AC source is recovered, the coil 7a starts magnetizing the electromagnet but the drill motor 9 remains deenergized with the relay 8 being opened unless the drill start switch 5 is pressed once again. However, such a controlling circuit of the conventional drilling machine requires the stop switch 11 for canceling the closing of the relay 8 in addition to the main switch 4 and the drill start switch 5. As the three switches in total are installed, they will be troublesome in control operation and also increase the overall cost.
If the coil 7a of the electromagnet has a fault such as disconnection or shortcircuit, the magnetization of the electromagnet is interrupted. The disconnection in the coil 7a may be detected through monitoring the flow of current in the coil 7a, which allows the drill motor 9 to be stopped immediately. It is however difficult to locate any shortcircuit in the coil 7a or, if a double coil type magnetizing system is employed, to identify disconnection in only one of the two coils connected in parallel.
It is an object of the present invention to provide a drill controlling apparatus which incorporates a simple switching system for controlling the restart of an electric drill upon the recovery from power failure and responding to any fault in an electromagnet used for securely anchoring the electric drill.